As I recall, around D-Day, the Nazis got their more vicious officers off the islands because they were needed on the mainland.
The lower ranked guys who replaced them actually worked with the locals to preserve the heritage dairy cattle breeds named after the two main islands despite considerable hardship in doing so.
Used to be a breed of dairy cattle named after Alderney, too, but it was absorbed into the Guernsey breed during that time.
Island at War (2004) I started watching this a couple years back, didn’t finish as the Wife was less interested. I need to give it another go if I can find it streaming.